The invention relates to a device and a method, with which, for optical inspection of a flat surface of an object, its plane-parallel orientation to a focus plane of a lens belonging to a measuring head can be achieved. For optical inspection of objects or of object surfaces, such as for example the surface of wafers, the relevant optic features a significant magnification. Consequently the depth of field is relatively small. If this depth of field is smaller than the distance error which can occur for the distance between lens and object this leads to partly unsharp images. It is precisely in the inspection of large surfaces of an object that these distance tolerances arise to a greater extent. These distance errors are produced overall by plane-parallelism errors in the inspection table, object holder, for example chuck, and object in relation to the focus plane of a lens of a measuring head.
Previously known systems are essentially designed for the inspection of small surfaces of an object. In this case the image is focused using autofocus operation of a microscope of an inspection unit. Another option is to move the lens into the focused position. This involves fitting an adapter ring with an adjusting element between the lens and the microscope body. The adapter ring allows the distance between object and lens to be adjusted in such a way that the image is focused. A further option is to move the microscope manually, using an adjusting wheel for example, with the distance between lens and surface of an object being corrected so that the overhead view of the object plane is displayed sharply in the eyepiece or the camera chip of a camera connected downstream from the optics. For inspection of smaller object surfaces one-off focusing is sufficient as a rule.